The present invention relates to semiconductor laser devices providing high power coherent radiation and, more particularly, relates to such devices utilizing frequency selective feedback type laser diodes integral with optical amplifiers having a diverging active region.
Conventional narrow stripe lasers are capable of producing good laser mode characteristics but are limited in their output power due to facet damage effects and heating effects at high input currents. Wide stripe semiconductor lasers, which provide high power, produce poor mode characteristics. The present invention overcomes these problems by providing an integrated semiconductor laser device which combines a frequency selective feedback laser and a power amplifier and is capable of emitting high power laser radiation with good mode characteristics.
A frequency selective feedback type laser diode is understood to be any diode laser which emits energy having a single longitudinal mode and in which the mechanism for producing the internal optical feedback within the device, as required for lasing, is that of a periodic structure along the length of the diode. This periodic structure can be effected by various approaches and the present invention is not limited to any single approach. One such approach is to generate a periodic grating within the diode. See, Integrated Optics, Vol. 7, T. Tamir (ed.), E. Gamire, "Semiconductor Components For Monolithic Applications". Another possible approach for generating a periodic structure is that of periodic current confinement. See, U.S. Pat. Nos. 4,445,218 and 4,359,776. Other approaches are known in the prior art in which a periodic structure is etched, diffused, electrically produced or otherwise achieved such that feedback and, consequently, lasing occurs.
Also, known are various types of semiconductor optical amplifiers such as those described in Semiconductor Optical Amplifiers, S. Kobayashi and T. Kimura, IEEE Spectrum, May 1984, pp 26-33. Prior art amplifiers, however, have found application in areas, such as communication, where very weak signals are restored to their original strength, and no attempt is made to achieve high power.
The present application is related to U.S. patent application Ser. No. 780,879, U.S. Pat. No. 4,713,821 entitled "Semiconductor Laser and Optical Amplifier", filed on Sept. 27, 1985, and assigned to the same assignee as the present invention.
The aforementioned application discloses a semiconductor light generating device which comprises a laser diode (LD) having an output facet cleaved apart from, and optically coupled to, an optical power amplifier (OA). The OA is formed so that its active region diverges in the forward direction. The laser beam diverges by diffraction as it leaves the LD and expands within the medium of the diverging active region of the OA so that the output laser beam is distributed over a wider area of the output facet of the 0A than would be the case without the divergence. Pumping current injected into the OA establishes the gain, or amplification, required to maintain constant optical power density within the amplifier active region from the entrance facet to the exit facet, eliminating both gain saturation and facet damage.
The above-described application overcame two serious problems associated with semiconductor diode optical amplifiers: gain saturation and degradation and catastrophic failure of the facet.
A possible problem that the invention in the above-referenced application has is presented by the cleave between the LD and the OA. The active area in both the LD and 0A must be of the same thickness and co-planar. In practice these requirements may be difficult to achieve since two discrete devices must be mechanically mounted in close proximity with critical tolerances in order to achieve optical coupling. While it is possible to cleave and couple laser devices, each unit must be cleaved individually thus increasing the manufacturing time and the variability among the laser/amplifier units.
Our contribution to the art is to improve the above-described device by providing a monolithic semiconductor laser device which combines a frequency selective feedback laser integral with an optical amplifier having a diverging active region. The LD and the OA are optically and physically coupled and the device is capable of producing coherent radiation at high power with good mode characteristics. As with the device disclosed in the co-pending application the present invention overcomes gain saturation and degradation of the amplifier and catastrophic failure of the facets.